Boss salutes battling Addicks
Goals in either half from Ji-sung Park and Darren Fletcher sealed the points for United, but the visitors served up a few scares throughout the afternoon.
"It was hard work for three points," Sir Alex told MUTV. "There were periods when there was a stalemate about the game and you have to give credit to Charlton, I thought it was a really good performance by them.
"It was a surprising performance I must say, because when teams are at the bottom of the league they don't always have the confidence to play, and I thought Charlton did."
Alan Pardew's side almost drew level when Dennis Rommedahl's free kick was flicked fractionally wide of Tomasz Kuszczak's goal, shortly before Fletcher clinched the victory.
The Boss reflected: "They had a free kick that was glanced on and it must have missed the post by a matter of inches. And these are worrying moments because 1-0 means nothing really, you can lose a goal in a second.
"The one good thing was that I thought we did defend really, really well today. That's one of the best in quite a while."
Park and Fletcher replaced Michael Carrick and Cristiano Ronaldo for United, while Kuszczak replaced broken-nose victim Edwin van der Sar.
The opportunity to field those fringe players was a major plus point for the Gaffer, with only 11 games remaining in the Premiership season.
"That's why you've got a squad," he said. "I keep stressing the point that the more miles we get on the clock of our pool players, it's going to benefit us at the right time of the season.
"I think we're playing well enough, I don't think we should always be looking over our shoulders. We should start looking forward ourselves and look at the exciting games ahead."